The invention relates to an improved passive super-twist nematic (STN) liquid crystal display (LCD) and, in particular, to normally white LCDs (referred to as optical mode interference or OMI displays). This is in contrast to the colloquial use of the term "film-compensated STN" or "FSTN" which refers to a type of normally black super-twist nematic liquid crystal display. Super-twist nematic liquid crystal displays are well-known in the art as described in, e.g., Scheffer and Nehring, Vol. 1, B. Bahadur, ed., World Scientific, pp. 231-274, 1990.
Both OMI and FSTN displays are examples of comparatively low-cost displays that can be driven by passive, i.e., multiplexed, addressing techniques (in contrast to the well-known active-matrix type of liquid crystal display).
2.1 Some Advantages and Disadvantages of OMI Displays
It is well-known in the art that an OMI display has an inherently wider operational temperature range than does a FSTN, e.g., Moia et al., "High-Contrast and High-Information-Content Optical-Mode-Interference (OMI) LCD with Video Response: Comparison with STN-LCDs," SID 93 Digest, Seattle, pp. 368-371, 1993. A wide operational temperature range is important in many applications, e.g., automobile dashboards, avionics system displays, hand-held portable electronic devices, and automated-controller applications such as gasoline pumps, manufacturing process controllers, and the like. It has also been reported that OMI displays exhibit faster on and off response times than do FSTN displays, e.g., see Moia et al.
A substantial drawback of conventional OMI displays, however, is that the display's viewing angle in the horizontal direction is too narrow for many applications in which a high operating-temperature range is desired. Also, a chromaticity of the white and black states varies with viewing angle and temperature.
2.2 O-Plate Compensation Technology
To improve the field of view and chromaticity stability, a birefringent O-plate compensator can be used. The O-plate compensator principle, as described in U.S. patent application Ser. No. 223,251 utilizes a positive birefringent material with its principal optic axis oriented at a substantially oblique angle with respect to the plane of the display (hence the term "O-plate"). "Substantially oblique" implies an angle appreciably greater than 0.degree. and less than 90.degree.. O-plates have been utilized, for example, with angles relative to the plane of the display between 30.degree. and 60.degree., typically at 45.degree.. Moreover, O-plates with either uniaxial or biaxial materials can be used. O-plate compensators can be placed in a variety of locations between a LCD's polarizer layer and analyzer layer.
In general, O-plate compensators may also include A-plates and/or negative C-plates as well as O-plates. As is well known in the art, an A-plate is a birefringent layer with its extraordinary axis (i.e., its c-axis) oriented parallel to the surface of the layer. Its a-axis is thus oriented normal to the surface (parallel to the direction of normally incident light), leading to its designation as an A-plate. A-plates may be fabricated by the use of uniaxially stretched polymer films, such as polyvinyl alcohol, or other suitably oriented organic birefringent materials.
A C-plate is a uniaxial birefringent layer with its extraordinary axis oriented perpendicular to the surface of the layer (parallel to the direction of normally incident light). Negatively birefringent C-plates may be fabricated by the use of uniaxially compressed polymers (See, e.g., Clerc et al., U.S. Pat. No. 4,701,028), stretched polymer films, or by the use of physical vapor deposited inorganic thin films (See, e.g., Yeh, et al., U.S. Pat. No. 5,196,953), for example.